Blender Camera Settings Millimeters/Field of View conversion
It's because the sensor's size plays a role either. Wikipedia says:


  So a standard 50 mm lens for 35 mm photography acts like a 50 mm standard "film" lens on a professional digital SLR, but would act closer to an 80 mm lens (1.6 x 50mm) on many mid-market DSLRs, and the 40 degree angle of view of a standard 50 mm lens on a film camera is equivalent to a 28–35 mm lens on many digital SLRs.


Hence, you need to increase the sensor's size to 42.447 
I believe there's a formula for this.